Observing Reactions

Observing Reactions

Description

Two seemingly identical sets of chemicals are provided for student study. Students observe systematically the results of combining the chemicals. When sharing the results with an entire class, debate regarding the differences in observations often ensues.

Hazards

1 M HCl is corrosive. Lead nitrate is toxic.

Precautions

Avoid ingestion of or contact with the chemicals.

Procedure

Place an acetate sheet over the viewing matrix.
Obtain a set of chemicals (4 bottles and a vial).
For each column, place a drop of the liquid or a piece of the solid on the area indicated according to the column label at top of the viewing matrix. Each rectangle is printed to be half-white and half-black to enhance the viewing of the chemicals. Colored materials show up better against the white background, while white and light-colored solids are seen more easily against the dark background. Place drops of the chemicals right on a light/dark edge.
For each row, place a drop of the liquid or a piece of the solid on the area indicated on the side of the viewing matrix. (Avoid contamination of the droppers. Wipe the forceps if they touch a solution.)
Make careful observations, and record them in the data table. 96-well plates may be used instead of the acetate sheets.
If this procedure is followed, you will end up mixing two samples at a time and will systematically make all of the possible combinations. (Order of addition does not matter for these chemicals.)
Carefully record the appearance of each mixture.

Handout

Handout Makeup

See Handouts above.

Name ___________________________ Class _______

Teacher __________________________

DoChem005 Observing Reactions

Record your observations of the reactions on the attached data sheet.
What is the best way for observations to be checked?
Why do the drops stay in one place on the plastic instead of running all over?
What happens if the plastic is placed on tilted table or surface?
Do you see any differences?
Can you explain the results in a logical way?
How would you plan an experiment to resolve any inconsistencies?

Teachers Guide

Purposes

To make observations of reactions.
To organize data in tabular form.
To draw conclusions from observations.
To defend conclusions in a debate.

Materials

Two sets of chemicals which appear to be alike. Half of the students receive one set while the other students receive the other set.
Set One: 10 dropper bottles labeled A to D containing:
- A. 10 mL 1 M hydrochloric acid (4.3 mL concentrated HCl diluted in water to 100 mL)
- B. 10 mL 0.1 M copper sulfate pentahydrate (2.5 g dissolved in water to 100 mL)
- C. 10 mL 0.1 M potassium iodide (1.7 g dissolved in water to 100 mL)
- D. 10 mL 0.1 M lead (II) nitrate (3.3 g dissolved in water to 100 mL)
- E. A few grams of mossy zinc metal (in a screw-cap vial)
Set Two: (A similar group of materials).
- A. 10 mL of 1 M hydrochloric acid solution
- B. 10 mL bromcresol green solution (0.01 g diluted with a 0.001 M sodium hydroxide solution to 100 mL)
- C. 10 mL of 0.1 M potassium iodide
- D. 10 mL of 1 M ammonium hydroxide (7.5 mL concentrated ammonia diluted in water to 100 mL)
- E. A few grams of zinc metal (in a screw-cap vial)
10 acetate sheet to place over viewing matrix
10 data sheet for recording student data
10 viewing matrix
10 tweezers
(Use of solutions stored in Beral pipets is encouraged; 96-well plates may be used.)

Lab Hints

The acetate sheet is an inexpensive and readily available alternative to expensive porcelain spot plates. Markings need not be made on the acetate; instead, a suitably-marked sheet (a data sheet) is placed underneath the acetate.
Plastic dropping bottles with dropping devices in the caps are best. If these bottles are not available, describe to students the techniques needed to avoid contamination of droppers. (A dropper tip should not touch anything except the solution it is being used to deliver. Only one dropper should be out at a time. Droppers should not touch plastic sheets.)
The bromcresol green solution in Set Two must match the color of the copper sulfate solution in Set One.
This experiment lends itself to microscale implementation using 96-well plates or acetate sheets and modified Beral pipets.

Time

Teacher preparation: 50 minutes

Presentation: 20-30 minutes

Hazards

1 M HCl is corrosive. Lead nitrate is toxic.

Disposal

Collect the used chemicals into a single vessel. Treat with 1 M sulfuric acid and allow to stand for several days. Filter the solid lead sulfate, and hold for solid waste disposal. Pour the supernatant liquid into the sink and flush with 10 volumes of tap water per volume of supernatant fluid.

Presentation?

Presentation Question:

What is the best way for observations to be checked?
- This is a question without a specific answer. In this case, switching sets of bottles will be effective. When the instructor uses one of the sets at the overhead projector, students who used the other set will probably be provoked to comment. Some students actually will change their report of what they saw.
Why do the drops stay in one place on the plastic instead of running all over?
- Surface tension holds the drops in place.
What happens if the plastic is placed on tilted table or surface?
- Nothing happens if the tilt is slight; surface tension is strong enough to hold them in place. The drops run if the tilt is pronounced.

Sample Data

Set One Sample data set:

Set Two:

Closure

Use an overlay on the overhead projector in front of the class. Ask students to provide the observations. After a few minutes there will be considerable heated debate. See if you can then develop two charts such that the students find their results compatible with one or the other.

The discussion can lead to ways in which scientists work out their differences. For example, do students suggest that they return to repeat experiments? Do they want to watch one another repeat experiments?

Discussion

In this experiment all students receive what appear to be identical groups of chemicals. Some students tend to disregard their observations and adopt the majority opinion. This experiment can help to build a student's confidence in the reliability of their observations.

Makeup Answers

1-3 see presentation questions.
4. All three students have different results. Student 2 & Student 3 have blue precipitates in the reaction grid. Student 1 & Student 3 have bright yellow precipitate in the reaction grid.
5. The tubes have not simply been mixed up because precipitates are observed in one that are not even present in the other. The solutions must contain different substances or compounds.
6. The student 1 and student 2 might interchange bottles and repeat the experiment to confirm the first result.

Key Words

discrepant event
observe
observation
model